The present invention relates to generally a yaw system for a windmill, the windmill comprising a tower and a nacelle, the tower and the nacelle being joined by the yaw system and the yaw system further comprising a bearing fixed to the tower, on which bearing the nacelle rests and slides in a yawing movement, and at least one yaw motor arranged to allow the nacelle to perform a rotary motion along the bearing. The invention also relates to a method for controlling the yaw of a windmill, comprising the steps of determining a set point for the windmill, determining a yaw error based on the set point and a current alignment of the windmill, determining a size and direction of a torque based at least on the yaw error, and applying the torque to at least one yaw motor of a yaw system for turning the turbine.
When using a windmill to generate electrical energy, it is in most cases desired to position the turbine perpendicular to the wind direction, or as close to a perpendicular position as possible. If the direction of the wind changes in such a way that the turbine is no longer perpendicular or close thereto, but rather parallel to the wind direction, a significant amount of energy is lost since the ability of the wind to cause a rotation of the blades is decreased. Also, an unsuitable wind direction causes an undesired increase of the load on the windmill and this can result in increased wear and tear, as well as an increased risk of serious damages to the components of the windmill.
It is previously known to attempt a solution to this problem, such as that shown by EP 1 571 334 (Gamesa Eolica), where a wind turbine yaw system is used to rotate the nacelle of a windmill in order to keep the turbine facing towards the wind. In order to accomplish this, a sensor detects the direction of the wind in relation to that of the nacelle onto which the turbine is mounted, and if the yaw error, i.e. the difference between these directions is too large, a yaw motor can interact with a gear ring in order to rotate the nacelle with the turbine around the tower of the windmill. When the yaw motor is not in operation, a set of yaw brakes are used to keep the nacelle in the desired position.
In order to use the yaw motor, the applied yaw brakes will have to be loosened, and the start of such a moving operation can be difficult to achieve without causing a sudden starts or undesired vibrations. One common way of avoiding this is to keep using at least one of the brakes also during use of the yaw motor, in order to provide for a more controlled motion. This requires a power of the yaw motor that is greater than that otherwise needed for the motion in itself, had no brake been applied.
Also, since the yaw system is designed to keep the nacelle fixed (stiff) in a position until a large enough yaw error is detected, all external loads are carried by the yaw brakes and the overall structure. The size of these loads is often unknown and this fact, together with the changes that occur in the loads as the strength of the wind rapidly changes, can lead to damages to the turbine and the windmill and especially to the yaw brakes as the loads are transferred to stresses in the yaw system material.
Similar systems are also shown by U.S. Pat. No. 7,436,083 (Shibata et al.) and by JP 2006-281655 (Ebara Corp.), but no reliable solutions to the problems described herein are disclosed.
There is therefore a need for a more reliable yaw system that can estimate the loads and reduce the wear and tear to the windmill and that can also increase the power generated from the windmill.